1. Field of the Invention
The present invention relates to an image transmission system, and in particular to an image transmission system which digitizes a plurality of image signals for the transmission.
In order to prevent occurrences of natural disasters such as floods, cliff crumbles, and mud flows or to quickly understand the situation after the occurrence of such disasters, it has been required to provide an image transmission system having monitoring cameras set at a plurality of monitoring points over dams, rivers, check dams, or the like from which picture signals are transmitted through communication lines as image data to be collected at such places as remote monitoring centers.
2. Description of the Related Art
The prior art of such image transmission system is shown in FIGS. 9 and 10.
FIG. 9 shows an analog image transmission system which transmits picture signals by 1:1 in the form of analog signals by using transmission medium such as optical fiber cables. The picture signals of cameras 10_1-10_3 are transmitted in the form of analog signals from one analog image transmitter to another, i.e. from 90_1 to 91_1, from 90_2 to 91_2, and from 90_3 to 91_3, respectively. The picture signals outputted from the analog image transmitters 91_1-91_3 are transmitted to an image switch 800 where they are properly selected by switching to be projected on a monitoring device 900.
In contrast to such an analog image transmission system, there is an image transmission system which digitizes picture signals to perform a multiplex transmission using the ATM method. FIG. 10 shows an example of such system.
The picture signals of the cameras 10_1-10_3 are transformed into digital signals i.e. image data and further compressed by image compressing portions 20_1-20_3, respectively to be transmitted to a cell assembly portion (CLAD) 30 for assembling the image data to cells. The cell assembly portion 30 transmits the cells to an ATM switching portion 40. The ATM switching portion 40 transmits the cells to a transmission line 600 by a time division multiplexing method and the like.
Such a digital image transmission system can flexibly deal with communications including a low speed communication, a communication with less information, and a high speed communication of a wide band, since a higher speed processing is possible and the number of transmission cells can be varied according to the quantity of generated information.
In order to perform a more detailed monitoring operation using such image transmission system, it is desirable to increase the number of monitoring points for collecting many monitoring pictures.
However, since the digital image transmission system of the prior art transmits all of the image data continuously regardless of the characters of the data, a band xcex2 of the transmission line 600 through which plural image data are simultaneously transmitted requires a total xcex1 of those image data band or more.
Namely, provided the bands of each transmission line through which the compressed data from the cameras 10_1-10_3 are transmitted are a , b, and c, respectively in FIG. 10 and compared with their total xcex1 (=a+b+c), the band xcex2 of the transmission line 600 requires to be greater than or equal to xcex1 (xcex2xe2x89xa7xcex1). Therefore, it leads to the increase of the transmission band and the expansion of the communication equipment.
Also, when transmitting plural image data through the same transmission line, since the band of each of the image data is preset and this preset band is always occupied fixedly, not only the transmission band increases but also there is a possibility of transmitting information which is not necessarily required.
Moreover, being fixed to a preset value, the compression rate of the image data cannot be adjusted to an appropriate transmission rate or transmission quality (image quality) depending on situations.
For example, in case of river monitoring, since the water level of the river does not rise rapidly in a short time unless there is an influence by a downpour and such, image transmissions at a relatively low rate (low quality) may be allowed in a normal condition. The monitoring precision may be raised by being switched over to the image transmission at a high rate (high quality) after the forecast or beginning of rainfall.
However, since the above-mentioned prior art has a fixed compression rate, if the preset rate is low, a necessary monitoring precision cannot be achieved when there is a change of an event or a possibility of a change. On the contrary, if the preset rate is high, more information than required are to be transmitted in a normal condition, resulting in an inefficient use of the transmission band.
It is accordingly an object of the present invention to provide an image transmission system capable of efficiently utilizing the transmission band and maintaining an appropriate quality of image.
[1] For the achievement of the above object, the image transmission system according to the present invention comprises an image compressing portion for digitizing each of a plurality of image signals and further transforming the same into compressed image data, a cell assembly portion for assembling the image data to cells, a switching portion for switching the cells from the cell assembly portion, and a band controller for controlling the cell assembly portion so that a band of a transmission line has a value lower than a total band required for a simultaneous transmission of each of the image data.
Namely, as schematically shown in FIG. 1, picture signals taken by cameras 10_1-10_3 are digitized at image compressing portions 20_1-20_3 and transmitted as compressed image data to a cell assembly portion 30. The cell assembly portion 30 transmits image data cells to a cell switching portion 40 under the control of a band controller 100.
At this time, the band controller 100 can arbitrarily set a control condition in order for the total of plural image data cells not to exceed an available band in the transmission line.
Thus, appropriately setting the control condition of the band controller 100 enables the band xcex2 of the transmission line 600 to be maintained at a lower value than the total a of the bands required for transmitting each of th image data.
[2] Also, in the above present invention [1], the band controller may divide the image signals into groups and control the cell assembly portion in a time division manner for at least one of the groups.
Namely, the band controller may divide the plural image signals into, for example, a group requiring a continuous transmission and at least one group capable of an intermittent transmission, and control the cell assembly portion in a time division manner for the group capable of the intermittent transmission.
[3] Also, in the above present invention [1], a network controller for controlling the band controller through a network may be provided.
Namely, if the system is provided in a network of a ring type and such in which a plurality of communication nodes sharing a communication line, a network controller may be provided for generally controlling the band controller at each communication node. Controlling the band as mentioned above enables supervising the band application status in the entire network.
[4] Also, in the above present invention [1], a sensor for detecting a change of an event to control the band controller may be provided.
Namely, providing a sensor such as a motion sensor or a water level sensor which detects a change of an event enables the sensor to control the band controller based on the change of the event. The band controller performs the band control as mentioned above.
Thus, performing the band control of the transmission line having the change of the event as a trigger enables the band of the transmission line to be used in accordance with the real-time changes of the situations.
[5] Also, in the above present invention [1], the band controller may control a compression rate of the image compressing portion.
In the above present invention [1], the band controller controls the band by controlling the cell assembly portion to control the amount of cells flowing in the switching portion from the cell assembly portion.
By contrast, in the present invention [5], the band controller controls the amount of cells flowing in the cell assembly portion from the image compressing portion by controlling the compression rate of the image compressing portion. Namely, the band controller can control the band of the transmission line by controlling the compression rate of the image compressing portion, instead of the band control.
[6] Also, the image transmission system according to the present invention may comprise an image compressing portion for digitizing each of a plurality of image signals and further transforming the same into compressed image data, a cell assembly portion for assembling the image data to cells, a switching portion for switching the cells from the cell assembly portion, and a compression rate controller for controlling a compression rate of the image compressing portion so that a band of a transmission line has a value lower than a total band required for a simultaneous transmission! of each of the image data.
Namely, in the present invention, the compression rate controller directly controls the image compressing portions 20_1-20_3 shown in FIG. 1, instead of the band control at the cell assembly portion. The amount of cells coming into the cell assembly portions from the image compressing portions can be controlled by controlling the compression rates of the image compressing portions. Namely, the compression rate controller can perform the band control among a plurality of image signals by controlling the compression rates of the image compressing portions.
[7] Also, in the above present invention [6], a network controller for controlling the compression rate controller through a network may be provided.
Namely, if the system is provided in a network of a ring type and such in which a plurality of communication nodes sharing a communication line, a network controller may be provided for generally controlling the compression rate controller at each communication node. Controlling the compression rate as mentioned above enables supervising the band application status in the entire network.
[8] Also, in the above present invention [6], a sensor for detecting a change of an event to control the compression rate controller may be provided.
Namely, providing a sensor such as a motion sensor or a water level sensor which detects a change of an event enables the sensor to control the compression rate controller based on the change of the event. The compression rate controller performs the compression rate control as mentioned above.
Thus, performing the compression rate control of the compression rate controller having the change of the event as a trigger enables the band of the transmission line to be used in accordance with the real-time changes of the situations.
[9], [10] Also, in the above present invention [3] or [7], the network controller may include control information in an overhead portion of an ATM cell which flows through the network.
Namely, the network controller provided in the network can include the control information in the overhead portion of the ATM cell which flows through the network.
[11], [12] Also, in the above present invention [3] or [7], the network controller may include control information in an payload portion of an ATM cell which flows through the network.
Namely, the network controller provided in the network may include the control information in the payload portion of the ATM cell which flows through the network.